The present invention relates to a new method of tracking of the type known as "step-by-step" or "step-tracking" by telecommunications antennae in particular.
Satellite telecommunications antennae are movable about two axes at right angles. A position control device is used to ensure permanent aiming of the antenna in the direction of the satellite. The development of low-cost stations and the employment of more and more stable geostationary satellites has enabled the development of less sophisticated position controls known as having "step-by-step" tracking or as "step-tracking".
The tracking unit or position control device placed on the market hitherto by the Applicants, operates as follows:
a tracking receiver delivers a d.c. voltage proportional to the power of the beacon signal received from the satellite; hence this voltage is a maximum when the antenna is aimed in the direction of the satellite;
a unit known as the driving and control unit measures this voltage and controls a set of asynchronous motors which enable the antenna to be brought into the direction of the satellite in accordance with the following cycle:
(a) measure the output voltage of the receiver; PA1 (b) displace the antenna along one axis and in a predetermined direction for a measured time; PA1 (c) a new measurement of the output voltage of the receiver is taken; PA1 (d) compare the new measurement with the previous measurement; PA1 (e) decision: PA1 (f) a new measurement, a new comparison, a decision to continue displacement in the same direction as long as the voltage continues to increase; PA1 (g) when the voltage received has dropped, a decision to change the axis of displacement by 90.degree., a repetition along this orthogonal axis of the foregoing steps a) to f); when PA1 (h) a decision to stop tracking from the voltage received has dropped. PA1 (a) measure the output voltage of the receiver; PA1 (b) displace the antenna along a first axis, in a certain direction; PA1 (c) during the displacement, successive measurements of the voltage with successive comparisons of each measurement with the previous one; PA1 (d) when the last measurement is found that measurement which is less than the previous one, change of the direction of displacement along the same axis; PA1 (e) during the new displacement, successive measurements of the voltage with successive comparisons of each measurement with the previous one; PA1 (f) when the last measurement is found again that measurement which is less than the previous one, stop and then displace in the reverse direction along the same axis for a measured time, depending upon the driving speed of the antenna along this axis, then change the axis by 90.degree.; PA1 (g) repeat, along this orthogonal axis but with another measured time which is a function of the driving speed along this axis, the steps a) to f) with either stopping of the tracking or repetition of a complete cycle a) to g), the number of cycles of this type before stopping the tracking is programmable by the user.
if the voltage has increased, displacement along the same axis for the same measured time and in the same direction; PA2 if the voltage has diminished, displacement along the same axis for the same measured time but in the contrary direction;
Such a tracking method presents certain disadvantages:
the antenna is positioned at the end of the cycle in a position close to but not in the exact direction of the satellite since it is a drop in voltage which terminates the tracking;
this method is slow since the antenna is stopped during the measurement and hence it reacts incorrectly to external disturbances;
a tracking cycle includes numerous starts and stops of the motors, resulting in considerable wear of these parts, as well as in the driven mechanical structures.